Evolution in the black hole mass–bulge mass relation: a theoretical perspective

We explore the growth of super-massive black holes and host galaxy bulges in the galaxy population using the Millennium Run ΛCDM simulation coupled with a model of galaxy formation. We find that, if galaxy mergers are the primary drivers for both bulge and black hole growth, then in the simplest picture one should expect the mBH – mbulge relation to evolve with redshift, with a larger black hole mass associated with a given bulge mass at earlier times relative to the present day. This result is independent of an evolving cold gas fraction in the galaxy population. The evolution arises from the disruption of galactic disks during mergers that make a larger fractional mass contribution to bulges at low redshift than at earlier epochs. There is no comparable growth mode for the black hole population. Thus, this effect produces evolution in the mBH –mbulge relation that is driven by bulge mass growth and not by black holes.